GASTROINTESTINAL ABSORPTION OF METALS

Estimating gastrointestinal absorption remains a significant challenge in the risk assessment of metals. This presentation reviews our curre nt understanding of the gastrointestinal absorption of lead (Pb) to il lustrate physiological mechanisms involved in metal absorption, new ap proaches that are being applied to the problem of estimating metal abs orption in humans, and issues related to integrating this information into risk assessment. Absorption of metals can be highly variable in h uman populations because it is influenced by a variety of factors that include the chemical form of the metal, environmental matrix in which the ingested metal is contained, gastrointestinal tract contents, die t, nutritional status, age, and, in some cases, genotype. Thus, in ris k assessment models, gastrointestinal absorption is best described as a variable whose distribution is determined in part by the above multi ple influences. Although we cannot expect to evaluate empirically each of the above factors in human populations, we can expect to achieve a sufficiently detailed understanding of absorption mechanisms to devel op conceptual and, eventually, quantitative models of absorption that account for some aspects of individual variability. A conceptual model is presented of the physiological processes involved in the transfer of ingested metals from the lumen of the gastrointestinal tract to the blood circulation. Components of the model include delivery of the me tal to the site(s) of absorption; distribution of metal among intracel lular and extracellular ligands and transcellular and paracellular pat hways of transfer across the gastrointestinal tract epithelium. The ga strointestinal absorption of Pb is discussed in the context of this mo del.